This invention relates to electron-emitting materials for use in electrodes in discharge lamps, cathode ray tubes, plasma displays and fluorescent display tubes, and a process for preparing the same.
Nowadays the social concern about energy saving and resource saving is increasing. With respect to light sources for general illumination and displays, active efforts have been made for saving the energy used to operate them. For example, the replacement of incandescent lamps by compact self-ballasted fluorescent lamps featuring a high energy efficiency and a longer life is accelerated as well as the replacement of cathode ray tubes by liquid crystal displays featuring a lower energy consumption. Accordingly, the demand for fluorescent lamps is increasing since they are used not only as compact self-ballasted fluorescent lamps, but also as back light sources for liquid crystal displays. For the same reason, energy-saving electrodes having a high energy efficiency are demanded for cathode ray tubes, plasma display panels, and fluorescent display tubes.
In prior art fluorescent lamps, oxide electrodes based on BaO are generally used. Such electrodes are described, for example, in JP-A 59-75553. The BaO base oxide electrodes have a good electron emission function, but a high resistivity. If a greater current flow is conducted for increasing electron emission, the electrode is heated to a high temperature, which leads to a higher vapor pressure and allows more evaporation, resulting in a shorter life. Also, the preparation of BaO base oxide electrodes requires decarboxylation because they are prepared by conducting electric current across a tungsten coil coated with barium carbonate for converting the carbonate salt to an oxide. This process, however, tends to achieve decarboxylation to an insufficient extent. When the resulting electrode is used in a fluorescent lamp having a slender bulb, carbon dioxide gas is left in the lamp bulb, giving rise to such problems as discharge instability and extremely reduced luminance retention.
U.S. Pat. No. 2,686,274 discloses a rod-shaped electrode obtained by reducing a ceramic material such as Ba2TiO4 into a semiconductor. Ceramic semiconductor electrodes of this type, however, suffer from the problems of low thermal impact resistance, easy deterioration by sputtering with mercury or rare gas ions, and a low current density available.
With these prior art fluorescent lamp electrodes borne in mind, the inventors proposed an electrode of the structure having a ceramic semiconductor received in a cylindrical container with one end open and the other end closed and made a number of improvements in this electrode and a discharge lamp using the electrode. See JP-B 6-103627, Japanese Patent Nos. 2,628,312, 2,773,174, and 2,754,647, JP-A 4-43546, JP-A 6-267404, JP-A 9-129177, JP-A 10-12189, JP-A 6-302298, JP-A 7-142031, JP-A 7-262963, and JP-A 10-3879. These electrodes have the advantages of improved sputtering resistance, retarded evaporation, retarded deterioration, and a long lifetime. With respect to sputtering resistance and evaporation, however, further improvements are desired.
Besides the electrodes for fluorescent lamps and other discharge lamps, evaporation and deterioration by ion sputtering are outstanding problems for various electrodes utilizing an electric discharge by way of hot or cold cathode action, for use in, for example, cathode ray tubes, electron microscopes, plasma displays, and field emission displays. It is desired to extend the lifetime of these electrodes.
An object of the invention is to provide a novel and improved electron-emitting material having restrained evaporation during electric discharge and a high resistance to ion sputtering. Another object of the invention is to provide a process capable of mass-scale production of such an electron-emitting material at low cost.
In a first aspect of the invention, there is provided an electron-emitting material containing a first component selected from the group consisting of barium, strontium, calcium and mixtures thereof, and a second component selected from the group consisting of tantalum, zirconium, niobium, titanium, hafnium and mixtures thereof, as metal element components and also containing oxynitride perovskite.
Preferably, the electron-emitting material contains MIMIIO2N type crystals as the oxynitride perovskite wherein MI denotes the first component and MII denotes the second component. The electron-emitting material preferably satisfies the relationship:
0.8xe2x89xa6X/Yxe2x89xa61.5
wherein X and Y are molar ratios of the first and second components to the total of the first and second components, respectively. The second component may be partially present in the form of a carbide or nitride or both. The electron-emitting material may further contain as an additional metal element component at least one element M which is selected from the group consisting of Mg, Sc, Y, La, V, Cr, Mo, W, Fe, Ni, and Al, preferably in an amount of more than 0 mass % to 10 mass % calculated as oxide. Typically the electron-emitting material further contains at least one oxide selected from among MI4MII2O9, MIMII2O6, MIMIIO3, MI5MII4O15, MI7MII6O22, and MI6MIIMII4O18 type crystals wherein MI and MII are as defined above. The electron-emitting emitting material preferably has a resistivity of 10xe2x88x926 to 103 xcexa9m at room temperature.
In a second aspect, the invention provides a process for preparing an electron-emitting material as defined above, comprising the oxynitride forming step of firing a metal element component-containing material disposed in proximity to carbon in a nitrogen gas-containing atmosphere to create oxynitride perovskite, yielding the electron-emitting material.
The nitrogen gas-containing atmosphere preferably has an oxygen partial pressure of 0 to 5.0xc3x97103 Pa. Preferably, a nitrogen gas stream is used as the nitrogen gas-containing atmosphere and fed at a flow rate of 0.0001 to 5 m/s per unit area in a cross section perpendicular to the direction of nitrogen stream in a space proximate to the material to be fired. In one preferred embodiment, carbon is admixed with the metal element component-containing material so that the metal element component-containing material is disposed in proximity to carbon; or a firing furnace which is at least partially composed of carbon is used in the oxynitride forming step so that the metal element component-containing material is disposed in proximity to carbon; or the metal element component-containing material is received in a container which is at least partially composed of carbon so that the metal element component-containing material is disposed in proximity to carbon. Preferably, the metal element component-containing material contains a compound oxide. In one preferred embodiment, the metal element component-containing material is molded into a compact, which is fired in the oxynitride forming step to provide a sintered body of electron-emitting material; or the metal element component-containing material is applied to form a coat, which is fired in the oxynitride forming step to provide a film of electron-emitting material.
The process may further involve the step of pulverizing the electron-emitting material resulting from the oxynitride forming step, yielding a powder of electron-emitting material. In one preferred embodiment, the process further involves the steps of molding the electron-emitting material powder into a compact, and firing the compact in a nitrogen gas-containing atmosphere to form a sintered body of electron-emitting material while restraining decomposition of the oxynitride perovskite. In another preferred embodiment, the process further involves the steps of preparing a slurry of the electron-emitting material powder, applying the slurry to form a coat, and heat treating the coat to form a film of the electron-emitting material.